Fix cluster exception message when pg_config is not executable

Author: elprans

asyncpg/cluster.py

@@ -552,7 +552,8 @@ def _find_pg_config(self, pg_config_path):
             raise ClusterError('could not find pg_config executable')
 
         if not os.path.isfile(pg_config_path):
-            raise ClusterError('{!r} is not an executable')
+            raise ClusterError('{!r} is not an executable'.format(
+                pg_config_path))
 
         return pg_config_path